I've flown a piper cadet arrow. The airplane has an engine of 200 horsepower at 2700 RPM. According to the power setting table in the POH, the maximum power we can set is 75% horsepower. With 100% power, we could fly faster (more fuel would burn though). Why doesn't the manufacturer provide the 100% power setting?
100% power on a typical aircraft engine is only good for short durations like takeoff or emergency maneuvers. Sustained 100% power setting will destroy the engine.
Not to mention flying faster is not always a good thing. It means higher aerodynamic loads on the airplane. Especially when you hit some turbulence.